Power amplifier



July 4, 1933.

LII

NETWRK E. T. BURTON POWER AMPLIFIER Filed June 25, 193].

FIG. 1

lNVENTOR E. 7? BURTON A TTORNEV Patented July 4, 1933 UNITED s'rarssPATENT OFFICE EVERETT T. BURTON, OF MILLBURN, NEVT JERSEY, ASSIGNOR TOBELL TELEPHONE LABORATORIES, INCORPORATED, OF NEW YORK, N. Y., ACORPORATION OF NEW YORK POWER AMPLIFIER Application filed June 25,

This invention relates to electric wave repeating apparatus andparticularly to the use of electron discharge amplifiers for repeatingand amplifying signals for transmission.

An object of the invention is to compensate for the distortionintroduced in the outgoing signals by operating a vacuum tube amplifiercircuit to its full power output whereby the normal ratio of the outputimpedance of the tube and its connected circuit is departed from duringportions of the time.

It is well known in the art that if external impedance in the outputcircuit of a vacuum tube amplifier is greater than the plate resistance,the characteristic of the amplifier becomes nearly linear when theamplifier is operated at low signal amplitudes, thereby rendering theoutput signal waves practically distortionless. It is also generallyknown in the art that this reduction in curvature of the characteristicmay be enhanced by adding series resistance in the output circuit of thevacuum tube. When it is desired to operate a vacuum tube at high powerefiiciency, it is necessary that the load impedance be matched to theimpedance of the tube and that any added resistance be very small oreliminated entirely. The characteristic of the circuit will become moreand more curved up to a definite limitas this resistance is reduced,thereby introducing considerably distortion in the output signal wave.Heretofore, in order to insure markedly increased power output withoutthe accompanying distortion, amplifiers of the push-pull type wereemployed in conjunction with output transformers having balanced primarywindings, but the use of these transformers is not always desirable,especially in circuits where the output currents are of low frequenciesand high amplitudes. Even with the use of output transformers theconnection of a push-pull amplifier to a line having sides unbalancedwith respect to ground is a problem which may involve considerabledifficulties.

Several methods have been employed whereby the use of outputtransformers has been avoided and high power efliciency in the vacuumtube ampliler has been obtained when the plate circuit is worked overpractically 1931. Serial No. 546,743.

the whole characteristic. The distortion thus introduced in the signalwave by the considerable decrease in the vacuum tube output circuitimpedance is reduced by impressing a compensating voltage on either theinput or the output circuit of the amplifier. Those methods aredisclosed in U. S. Patent 1,856,- 373, granted to E. T. Burton on May 3,1932. In that patent a compensating vacuum tube is arranged with respectto the amplifier vacuum tube in the following ways: (1) It has its gridcircuit connected in parallel and its plate circuit connected in serieswith the grid circuit of the amplifier. (2) It has both its grid and itsplate circuits connected in series with the plate circuit of theamplifier.

According to the present invention, the desired compensation is effectedby connecting the respective grid circuits of the compensating and theamplifier vacuum tubes in parallel and their respective plate circuitsare connected in parallel. A portion of the latter circuits is connectedin a special manner to operate over a common path wherein compensationis effected.

A better understanding of the invention will be had from the followingdetailed description and appended claims when taken in conjunction withthe accompanying drawing in which:

Fig. 1 illustrates schematically an embodiment of the invention foreffecting positive current compensation to correct for distortioninherent in the production of even order multiple frequencies;

Fig. 2 represents the input potential-output current characteristic ofthe power amplifier circuit shown in Fig. 1.

Referring to Fig. 1 the signal input voltage is received over conductors11 and 12 from a generating or reproducing source 13 such as atransmitter, a repeater, or an amplifier, and impressed on the gridcircuit of a thermionic tube 14: say the third stage of a signal shapingamplifier circuit. Desired shaping of the signals in the output circuitof tube 14 is obtained by condenser 15 and a suitable network showndiagrammatically by block 16. The output of tube 14 is in turn impressedon the grid circuit of a thermionic tube or a banl: of thermionictubesshown within block 17 which represents the fourth and last stage ofthe amplifier circuit. Across the signal input circuit of the last stageis connected a potentiometer 18 to which is adjustably connected thegrid circuit of a compensator thermionic tube 19 so that both the inputcircuit of the last stage and that of the compensator tube, are adaptedto receive the signal input voltage. v

The plate circuit of the compensator tube 19 is connected in common withpart of the plate circuit of the amplifier tubes 17 in such a way thatwhen the compensating voltage is impressed on the amplifier voltage inthe impedance common to both circuits, the output plate current, inresponse to an incoming positive impulse, causes a decrease in theamplitude of the amplifier output current as described in the followingdescription of the operation. a

The operation of the. circuit arrangement shown in Fig. 1 will now bedescribed by referring to Fig. 2 which represents the inputpotential-output current characteristic of the amplifier circuit beforeand after compensation of the positive current impulses is effected tocorrect for dissimilarity of reproduction of positive and .negativeimpulses. The normal or uncompensated characteristic of the amplifiercircuit is represented by curve a b c of Fig. 2 which is indicative ofhigh distortion in the positive current side of the output signal waveof the amplifier circuit. Therefore, in order to reduce this distortionand cause the degree of amplification to be approximately the same forboth sides of the signal wave, the plate circuit of the compensator tube19 is arranged to include resistance 21 which constitutes a part of theplate circuit of the amplifier tubes 17. The plate circuit of amplifiertubes 17 includes the balancing resistance 22 in addition to the seriesor common resistance 21, the resistance 22 not being a part of the platecircuit of the compensator tube 19. The resistances 21 and 22 havepreferred values of approximately 2,000 and 500 ohms respectively withcertain types of commercial vacuum tubes.

The C potential furnished by battery 23 in the compensator grid circuitis so adjusted that when no signal input voltage is applied acrosspotentiometer 18 the compensating plate current is reduced to a low orzero value. In the case where a negative signal is applied, thecompensator tube grid is carried still further negative with nocompensating effect on the amplifier output circuit thereby causing theplate current of the latter to follow the portion ba of thecharacteristic. When an increasing positive voltage is applied to theinput, the compensator plate will draw current which will increase withthe input voltage.

The plate current of the compensator tube, due to an incoming positivesweep of the incoming signal wave, flows through a circuit extendingfrom the plate and filament of tube 19, conductor 24, battery 25,resistance 21, conductors 26, 27 and 28, resistance 29, back to theplateof tube 19. The plate current of the amplifier tubes 17, due to the sameincoming positive sweep of the incoming signal wave, flows throughparallel paths between the plates and filaments of the amplifier tubes,conductors 30 and 31, resistance 22,

batteries 32 and 25, resistance 21, conductor 26, conductors 27 and 33in parallel, back to the respective plates of the tubes 17. Thus theplate current of the compensator flows through resistance 21 which iscommon to the plate circuit of the amplifier tubes, but does not flowthrough resistance 22. This causes the output of the compensator tube toact on the output of the amplifier tubes to produce a decrease in thecurrent in the output circuit 20. By properly adjusting the constants ofthe compensator stage, the curve be ismade the inverse image of thecurve ba, thereby producing an input potentiahoutput currentcharacteristic, which shows the positive output current of the amplifiercircuit reduced to amplitudes symmetrical with thoseof the negativeoutput current. This type of compensator is known as a positivecompensator since it effects the positive sweeps only of the incomingsignal wave. Its effect is similar to the type of compensator circuitshown in Fig. 1 of the applicants U. S. Patent 1,856,373, supra.

What is claimed is:

1. An amplifier circuit, a space discharge device, a second spacedischarge device, said devices having curved amplificationcharacteristics and being so arrangedv with respect to each other thatthe input voltage is impressed on the first device and the total inputvoltage or any part thereof is simultaneously impressed on said seconddevice, an output circuit for each of said devices, the output circuitsbeing. so arranged with respect to each other that the currents in acommon portion of the output circuits are superimposed on each other insuch a manner as to effect in a portion of. the output circuit of thefirst device only, a characteristic having on opposite sides of itsoperating point, curves which are symmetrical but inversely disposed toeach other.

2. An amplifier circuit according to claim 1 wherein the output circuitsare connected in parallel with a portion of each, connected in common.

3. Anamplifier circuit according to claim 1 wherein an impedance elementis common to both of said output circuits and a second impedance elementis individual to the output circuit of the first mentioned dischargedevice whereby the current in said output circuits are superimposed oneach other so that the output current of one device effects a control onthe output current of the other device.

4. An amplifier circuit according to claim 1 wherein a resistanceelement is common to both of said output circuits and a secondresistance element is individual to the output circuit of said firstmentioned first discharge device whereby the currents in said outputcircuits are superimposed on each other for the purpose of effecting achange in the curva- EVERETT T. BURTON.

